The present invention relates to a light guide plate for use in a planar lighting device that may be used for backlight of a liquid crystal display.
A liquid crystal display uses a planar lighting device (a backlight unit) which illuminates a liquid crystal display panel by irradiation of light from the back side of the liquid crystal display panel. The backlight unit is configured using a light guide plate for diffusing light emitted from an illumination light source to illuminate the liquid crystal display panel and parts such as a prism sheet and a diffusion sheet for making outgoing light from the light guide plate uniform.
As a backlight unit that allows the thickness reduction, use is made of an edge-lit backlight unit using a light guide plate in which a light source is disposed on a lateral surface of the light guide plate so that light incident from the lateral surface is guided in a predetermined direction different from the incident direction and is allowed to exit from a light exit surface which is a front surface.
It is proposed to use, as a light guide plate for use in such an edge-lit backlight unit, a light guide plate in plate form which is obtained by kneading and dispersing scattering particles for light scattering inside in order to guide the light incident from the lateral surface (light incidence surface) toward the front surface side (light exit surface).
The degree of light scattering caused by scattering particles in a case where the scattering particles are dispersed inside the light guide plate to scatter light varies depending on the light wavelength. Moreover, light at a wavelength at which scattering is more likely to occur exits after being scattered more at a region closer to the lateral surface through which the light entered, whereupon the quantity of light that may reach a region far from the light incidence surface is reduced to relatively decrease the quantity of outgoing light from the region far from the light incidence surface. On the other hand, light at a wavelength at which scattering is less likely to occur is not easily scattered, and hence the quantity of outgoing light from a region near the light incidence surface is reduced, whereas the quantity of light that may reach a region far from the light incidence surface is increased to relatively increase the quantity of outgoing light from the region far away from the light incidence surface. Accordingly, there was a problem that the ratio of quantities of light at respective wavelengths included in outgoing light through the light exit surface varies depending on the distance from the light incidence surface, thus causing color unevenness in the outgoing light.
Under the circumstances, various propositions have been made to reduce color unevenness of outgoing light in a light guide plate containing scattering particles dispersed therein.
For example, JP 11-153963 A describes that a light scattering guide is provided with scattering ability so that the ratio between the scattering efficiency in a long wavelength region and that in a short wavelength region is adjusted in a range of 0.75 to 1.25. More specifically, it is described to reduce color unevenness by adjusting the scattering efficiency ratio in a range of 0.75 to 1.25 through adjustment of the particle size of scattering particles to about 7 μm.
WO 2007/058060 describes that the rear surface is subjected to light diffusion treatment using, for example, a dot pattern having gradation from a lateral end surface having a light source disposed thereon toward a direction further away from the light source such that the tone unevenness which is a difference between a maximum value and a minimum value of the yellowness as obtained from tristimulus values at 25 luminance measurement points on the light exit surface is adjusted to 20 or less.